When creating membrane features, move convolver object out of the loop.
Add junit dependency.
Add simple unit test for the Weka Segmentation.

Authored by Dscho in fiji/fiji/ commit 28d466512e35011e01f5201aedd30a3cd8e89dfd.
Add a more sophisticated test for the Weka Segmentation

(By dscho, from fiji/fiji/ commit 755e4f6c491d6061acf2a89f8c8192b05787b83d)
This test segments the Bridge example; unfortunately, it runs much
slower and needs an example image with known-good segmentation.
Therefore we provide a main() method that downloads the sample and
generates the known-good segmentation. The corresponding test is skipped
if either image is missing.